1. Field of the Invention
This invention relates to iron-rich metallic glass alloys having high saturation induction that evidence particularly superior soft ferromagnetic properties when subjected to high magnetization rates.
2. Description of the Prior Art
Glassy metal alloys (metallic glasses) are metastable materials lacking any long range order. They are conveniently prepared by rapid quenching from the melt using processing techniques that are conventional in the art. Examples of such metallic glasses and methods for their manufacture are disclosed in U.S. Pat. Nos. 3,856,513, 4,067,732 and 4,142,571. The advantageous soft magnetic characteristics of metallic glasses, as disclosed in these patents, have been exploited in their wide use as materials in a variety of magnetic cores, such as in distribution transformers, switch-mode power supplies, tape recording heads and the like.
Applications for soft magnetic cores, in a particular class that is now receiving increased attention, are generically referred to as pulse power applications. In these applications, a low average power input, with a long acquisition time, is converted to an output that has high peak power delivered in a short transfer time. In the production of such high power pulses of electrical energy, very fast magnetization reversals, ranging up to 100 T/.mu.s (or 100 MT/s), occur in the core materials. Examples of pulse power applications include saturable reactors for magnetic pulse compression and for protection of circuit elements during turn on, and pulse transformers in linear induction particle accelerators.
Metallic glasses are very well suited for pulse power applications because of their high resistivities and thin ribbon geometry, which allow low losses under fast magnetization reversals. (See, for example, (i) "Metallic Glasses in High-Energy Pulsed-Power Systems", by C. H. Smith, in Glass . . . Current Issues, A. J. Wright and J. Dupuy, eds., (NATO ASI Series E, No. 92, Martinus Nijhoff Pub., Dordrecht, The Netherlands, 1985) pp. 188-199.) Furthermore, metallic glasses, due to their noncrystalline nature, bear no magneto-crystalline anisotropy and, consequently, may be annealed to deliver very large flux swings, with values approaching the theoretical maximum value of twice the saturation induction of the material, under rapid magnetization rates. These advantageous aspects of metallic glass materials have led to their use as core materials in various pulse power applications: in high power pulse sources for linear induction particle accelerators, as induction modules for coupling energy from the pulse source to the beam of these accelerators, as magnetic switches in power generators, in inertial confinement fusion research, and in magnetic modulators for driving excimer lasers.
In a typical pulse power application, the core material is initially "parked" in, or biased into, a specific magnetic state through the imposition of appropriate external magnetic fields. For example, the application of a large, negative d.c. field will place the core material in a negatively saturated state. (The direction in which the core material will be driven into saturation during the application is referred to as the positive direction.) A subsequent removal of this field will position the core material at negative remanence. The former procedure allows for a maximum flux swing of twice the saturation induction in the core material but, as a matter of convenience, the latter procedure, known as the pulse reset, is most commonly employed. The maximum flux swing is then the sum of the remanent and saturation inductions. Henceforth, unless otherwise specified, the term "maximum flux swing", as used herein, connotes a value that is determined by the sum of the remanent and saturation inductions. Metallic glasses may easily be annealed to yield a value for B.sub.r, the remanent induction, that is very close to B.sub.s, the saturation induction. The input that is to be compressed, or transformed, in the application, is then applied to the core material.
Most pulse power applications require a high saturation induction in the core material, which leads to a large flux swing in the core. The core material should, preferably, also possess a low induced magnetic anisotropy energy. A low magnetic anisotropy energy leads to lower core losses, by facilitating the establishment of an optimal ferromagnetic domain structure, and therefore allow the cores to operate with greater efficiency. METGLAS.RTM. 2605CO (nominal composition: Fe.sub.66 Co.sub.18 B.sub.15 Si.sub.1), available from Allied-Signal Inc., is a high induction metallic glass alloy currently used in many of the pulse power applications recited above. This metallic glass is taught by U.S. Pat. No. 4,321,090, wherein metallic glasses having a high saturation induction are disclosed. The saturation induction of this glassy alloy is about 1.75 T. However, the high cobalt content in this alloy imparts a high value for the magnetic anisotropy energy and, consequently, high core losses. The value of about 900 J/m.sup.3 for the magnetic anisotropy energy in this alloy is among the highest obtained in metallic glasses. In spite of its high induction, a maximum flux swing of only about 3.2 T is attainable from this alloy. Furthermore, the high Co content in this alloy leads to high raw material costs. Considering that the cores used in pulse power applications may contain as much as 100 kg of core material per core, and considering that Co had been classified as a strategic material, a more economical alloy containing substantially reduced levels of Co is highly desirable.
A metallic glass alloy that contains no cobalt is METGLAS.RTM. 2605SC (nominal composition: Fe.sub.81 B.sub.13.5 Si.sub.3.5 C.sub.2), available from Allied-Signal Inc. This alloy is disclosed in U.S. Pat. No. 4,219,355. The low magnetic anisotropy energy (about 100 J/m.sup.3) of this alloy has been exploited in certain pulse power applications. However, the lower saturation induction (about 1.57 T) and a correspondingly lower maximum flux swing (about 2.9 T) available from this alloy have deterred widespread use of this alloy in pulse power applications.
A metallic glass alloy that offered a combination of high induction (large flux swings) and low magnetic anisotropy energy would be highly desirable for the purpose of pulse power applications. An additional advantage would be derived if such an alloy were to offer economy in production costs.